This invention relates to a drive system for a brushless dc motor, and more particularly to a drive system which avoids the degrading of torque uniformity due to variation of Hall generator characteristics when used as rotor position sensors.
In prior engineering, there are two types of driving system for driving brushless dc motors having Hall generators as position sensors. One of these is a voltage control type and the other is a current control type. In the voltage control type, voltage across one armature winding is controlled so as to be proportional to the corresponding Hall generator output. The magnitude and the direction of torque is controlled by the current applied to the Hall generators. Torque is approximately proportional to the voltage across armature winding when the BEMF (back electromotive force) due to the rotation is small enough to make the armature voltage approximately proportional to the armature current. However when the output voltages of Hall generators differ from each other due to variations of Hall generator characteristics (with equal magnitudes of applied currents), torque uniformity is degraded because the magnitude of armature current differs in each phase winding.
Under the current type control, Hall generators are used to select the armature winding to be supplied with current according to the rotation of the rotor. Armature current directly proportional to torque is controlled so as to generate an adequate torque. Since the wave form of an armature winding is not sinusoidal but square, longitudinal force is generated and causes undesirable vibration and non-uniformity of torque in the case of a flat type motor where rotor magnet and the armature windings are faced to form an axial air gap. Besides, torque uniformity is degraded by the phase shift of current switching to select the armature winding caused by the variation of the output voltage among Hall generators.
A significant disadvantage of the conventional system mentioned above is the requirement either of means to adjust the output voltage of Hall generators or of selection of the Hall generators in order to improve the non-uniformity of torque.